In fact this is more an enhancement request than a real bug (in fact is has been designed to do just that) but as I didn't found a way to differentiate a file and a directory named core, to act differently upon them. Ignoring core files but not core directories would be a saner default behaviour.

a lot of projects use directories named "core" and those should not violate default naming conventions as they are not core files of crashed programs but directory containing source files for the "core" of the project.